1. Field of the Invention
The present invention relates to the field of turbine technology. It concerns a damping arrangement for a blade of an axial turbine.
2. Brief Description of the Related Art
The rotor blades of axial turbines, for example gas turbines, extend in the radial direction and are mounted on the outer periphery of the rotor which rotates about the rotor axis. They can be configured as free-standing blade aerofoils in a turbine stage but can also be connected to one another on their outer periphery by a shroud, a snubber or a lacing wire.
During the operation of the turbine, the blades are excited to vibrate by the interaction with the medium flowing at high velocity through the turbine and by the rapid rotation; loads are imposed on the blades and bearings and have a disadvantageous effect on the efficiency. Various proposals to damp the vibrations occurring in the blades have, therefore, already been made in the past. Some of these proposals are limited to providing special damping elements in the region of the blade fastening. Although this has the advantage that no modifications have to be undertaken on the blade aerofoil itself, it has the disadvantage that vibrations in the blade aerofoil can only be influenced very indirectly.
In other proposals, therefore, there has been a movement toward arranging and accommodating damping elements in the blade aerofoil itself, which elements influence the vibrations directly within the blade aerofoil. As an example, a damping arrangement for the rotor blades of a turbine is known from EP-B1-0 727 563, in which arrangement a circular recess or pocket is arranged in the outer third of the blade aerofoil at the center of the pressure surface, which pocket accommodates a (sinusoidal) damping element. The pocket is closed toward the outside by a pocket cover matched to the blade aerofoil contour. The damping element is in contact with the inner walls of the pocket and the pocket cover at some locations and, by means of its movement relative to the blade, damps the vibrations occurring in the blade aerofoil by friction on the walls. Disadvantageous features of this known damping arrangement are the complicated construction of the pocket and pocket cover and the spatial limitation of the damping arrangement to a very small region of the blade aerofoil.
On the other hand, the solution proposed in U.S. Pat. No. 6,607,359 manages without a cover. In this solution, a simply shaped damping element is inserted in such a way into a specially prepared pocket that the outwardly directed surface of the damping element seamlessly continues the pressure surface of the blade aerofoil. This solution also requires complicated and accurate processing of the individual elements of the damping device and only permits locally limited damping.
In WO-A1-01/49975, a narrow air chamber is formed on the suction surface of the blade by generating a recess and subsequently covering by (welding on) a cover. This has, due to the viscosity of the moving air, a damping effect on the blade vibrations which occur. This type of damping is not very flexible and likewise requires complicated and accurate processing and/or reworking of the blade surface in order to avoid aerodynamic disadvantages due to the installation of the damping device.
Summarizing in the case of the solutions mentioned above, it can be stated, on the one hand, that complicated and careful machining is necessary because of the action on the aerodynamically important blade surface and, on the other, that damping can only be realized in narrowly limited regions.